The present invention relates to image forming apparatuses, such as digital copying machines, optical printers and other apparatuses capable forming images, and an optical scanning apparatus included in such image forming apparatuses.
An optical scanning apparatus is known for use in image forming apparatuses such as digital copying apparatuses, optical printers and other such apparatuses. Optical scanning in an optical scanning apparatus is performed by scanning a beam spot by optically forming the beam spot on a surface to be scanned. The diameter of a beam spot is an important factor influencing the quality of an image generated by the scanning process. In order to produce a good quality image, a scanning beam has to be adequately condensed onto the surface to be scanned. Because an optical scanning apparatus generally includes movable portions such as a rotating polygonal mirror for deflecting a light beam from a light source toward the surface to be scanned, the arrangement conditions of an optical system of the optical scanning apparatus is slightly changed by the influence of vibrations or other external environmental factors over time. Moreover, when an optical system includes a plastic lens or similar lens elements which is easily influenced by temperature and humidity, the characteristics of the optical system slightly fluctuate according to changes in the operating environment of the optical scanning apparatus.
If such optical arrangement deviation and optical characteristic fluctuation occurs, the light condensing position (focusing position) of a scanning beam deviates relative to the surface to be scanned, and thereby, the beam spot diameter increases.
Therefore, several attempts have been made to correct the deviation of the focal position of a scanning beam occurring over time or caused by environmental fluctuations.
For example, Japanese Patent Application Laid-Open No. 10-20225 describes an optical scanning apparatus for condensing a laser beam radiated from a laser beam source into an optical beam spot and linearly scanning a surface to be scanned at a substantially constant velocity with the optical beam spot. The optical scanning apparatus includes a focusing lens for adjusting the condensing position of a laser beam radiated from the laser beam source, a detecting device for detecting the passage of a scanned laser beam to generate a detection signal, a pulse emitting device for allowing the laser beam source to emit a pulse at a predetermined time after the generation of the detection signal, a beam condensed state detecting device located at a position that is substantially optically equivalent to the position of the surface to be scanned for detecting the beam condensed state using a knife edge process, and a control device for driving the focusing lens based on the detection result of the beam condensed state detecting device to adjust the condensed position of the laser beam.
Moreover, another optical scanning apparatus is described in Japanese Patent Publication No. 2761723, which includes a light source for emitting a laser beam, a collimator lens for collimating the laser beam emitted from the light source, a photoelectric converting element for receiving a laser beam passed through the collimator lens, and an adjusting device for adjusting the position of the collimator lens in an optical axis direction in accordance with the output of the photoelectric converting element. The adjusting device adjusts the position of the collimator lens in accordance with a difference between a maximum output value and a minimum output value of the photoelectric converting element when receiving the laser beam flickering in response to an image signal.
In the optical scanning apparatus described in Japanese Patent Application Laid-Open No.10-20225, the laser beam source is allowed to emit a pulse from a pulse emitting device, and the beam condensed state is detected by the beam condensed state detecting device located at a position that is substantially optically equivalent to that of the surface to be scanned. However, a pulse emitting timing of the pulse emitting device actually deviates. Therefore, the detection accuracy of the beam condensed state is decreased when such deviation in pulse emitting timings occurs. Moreover, because the beam condensed state is detected by the beam condensed state detecting device using a knife edge process, the beam condensed state of a main scanning direction can be detected, but the beam condensed state of a sub scanning direction cannot be detected.
There is no apparatus or method for detecting the beam condensed states independently in the main scanning direction and in the sub scanning direction in the conventional art.
Moreover, in the optical scanning apparatus described in the Japanese Patent Application Laid-Open No. 10-20225 or Publication No. 2761723, an adjusting mechanism for adjusting a focal position adjusts the focusing lens or the collimator lens in the optical axis direction. Accuracy is required for the positional relationship between the light source and the focusing lens or the collimator lens. Also, an optical axis deviation or the like occurs when the focusing lens or the collimator lens is displaced for adjusting the focal position. Therefore, it is actually difficult to adjust the focusing lens or the collimator lens.
Furthermore, even when the focusing lens or the collimator lens is adjusted in the optical axis direction, the focal position cannot necessarily be corrected in the main and sub scanning directions at the same time. Particularly, in an optical scanning apparatus, the magnification of an optical system disposed on the optical path leading to a surface to be scanned from a light source often differs in the main scanning direction and in the sub scanning direction, for example, because of surface tilt correction for a rotating polygonal mirror functioning as a deflector for deflecting a light beam from a light source toward a surface to be scanned. In this case, even if the position of a collimating lens is adjusted in the optical axis direction, the focal position deviation of the main scanning direction and sub scanning direction cannot simultaneously be corrected.
The prior art method which includes using a knife edge process can only detect a beam condensed state in the main scanning direction and can not detect the beam condensed state in the sub scanning direction. Also, only a single pulse at a time is detected in the knife edge process so that the detection accuracy is relatively low. Further, the knife edge process requires the use of at least two pixels to achieve detection of the beam condensed state.
The other conventional apparatus which uses a line CCD for detecting a beam condensed state on a surface to be scanned is capable of more accurate detection of the beam condensed state, but is very expensive due to the cost of the line CCD and requires many pixels to achieve the detection. Furthermore, the line CCD can only detect the beam condensed state in the sub scanning direction and can not detect the beam condensed state in the main scanning direction.
Another conventional method uses an area CCD to detect the beam condensed state. Similar to the line CCD, the area CCD is very expensive and requires even more pixels to detect the beam condensed state. Also, the area CCD uses only a single pulse to detect the condensed state of the beam spot which has limited detection accuracy.
In order to overcome the above-described and other problems, preferred embodiments of the present invention provide a novel optical scanning apparatus which greatly improves the detection accuracy of an image forming state of a light beam.
The preferred embodiments of the present invention further provide a novel optical scanning apparatus which more accurately detects the location of a waist position of a light beam using an inexpensive element.
The preferred embodiments of the present invention further provide a novel optical scanning apparatus which detects the location of a waist position of a light beam using an inexpensive element independently in the main scanning direction and the sub scanning direction.
The preferred embodiments of the present invention further provide a novel optical scanning apparatus which detects the location of a waist position of a light beam using a single pixel which is less expensive than prior art apparatuses and using a continuous pulse which is more accurate than the single pulse process used in the prior art, to accurately and precisely detect the waist position in the main scanning and the sub scanning directions for more accurate and complete information about the waist position.
Further, preferred embodiments of the present invention provide a novel optical scanning apparatus which prevents deterioration of a detection accuracy attributed to a light quantity dispersion on a detecting device by a reflectance dispersion of each surface of a polygonal mirror.
Other preferred embodiments of the present invention provide a novel optical scanning apparatus which easily and securely corrects the above-described focal position deviation of a scanning beam attributed to environmental changes, and other influencing factors.
According to a preferred embodiment of the present invention, an optical scanning apparatus includes a light source for radiating a light beam, an optical scanning system for deflecting the light beam from the light source and condensing the light beam on a surface to be scanned, a detecting device for detecting an image forming state of the light beam scanned by the optical scanning system, and an adjusting mechanism for adjusting the focal position of the light beam on the surface to be scanned. While the focal position of the light beam is changed continuously or at a predetermined pitch by the adjusting mechanism, the detecting device monitors the image forming state of the light beam and detects the location and vicinity of a light beam waist position relative to a desired position on the surface to be scanned.
According to another preferred embodiment of the present invention, in the above-described optical scanning apparatus, the adjusting mechanism includes an adjusting device for adjusting the focal position of the light beam independently in at least one of a main scanning direction and a sub scanning direction.
According to another preferred embodiment of the present invention, in the above-described optical scanning apparatuses, the detecting device detects the vicinity of the waist position of the light beam independently in the main scanning direction and the sub scanning direction.
According to another preferred embodiment of the present invention, in the above-described optical scanning apparatuses, the detecting device is configured to have an opening in the main scanning direction, and the opening may be inclined by using the light beam incident upon the detecting device as a rotation axis.
According to still another preferred embodiment of the present invention, in the above-described optical scanning apparatuses, a deflecting device for deflecting a light beam from the light source includes a polygonal mirror, and the deflecting device uses a common deflecting surface in one cycle of detecting the image forming state of the light beam.
According to other preferred embodiments of the present invention, an optical scanning apparatus includes a light source, a coupling lens, an optical scanning system, and a correcting/adjusting device. The light source radiates a light flux for optical scanning. As the light source, a semiconductor laser can preferably be used. The coupling lens converts the light flux radiated from the light source to a parallel light flux or a converged light flux or a divergent light flux. The optical scanning system deflects the light flux converted by the coupling lens, and condenses the deflected scanning beam onto a surface to be scanned. Therefore, the optical scanning system has an optical deflector for deflecting the light flux from the coupling lens, and a scanning image forming optical system for condensing a scanning beam deflected by the optical deflector on the surface to be scanned. As the optical deflector, a rotating polygonal mirror, a rotating two-plane mirror, a rotating single-plane mirror, and other such mirrors can preferably be used. The scanning image forming optical system can be constituted by a single lens or a plurality of lenses, or by an image forming mirror, or by an image forming mirror and one or more lenses.
The correcting/adjusting device corrects and adjusts the focal position deviation of the scanning beam on the surface to be scanned caused by environmental fluctuations and other factors, and includes an optical correcting system.
The optical correcting system is preferably located between the coupling lens and a deflecting surface of the deflector in the optical scanning system, and has at least one anamorphic surface which has different power in the main scanning direction and in the sub scanning direction. The optical element constituting the optical correcting system can be constituted by a lens or a mirror.
The optical correcting system can also function as an optical system for correcting the surface tilt of the deflector and for forming a light flux from the coupling lens into an image substantially linear in the main scanning direction in the vicinity of a deflecting surface of the deflector. A dedicated optical system for correcting the surface tilt of the deflector other than the optical correcting system can be used.
According to another preferred embodiment of the present invention, in the immediately above-described optical scanning apparatus, the correcting/adjusting device can include a beam spot detecting device, a displacing mechanism, and a control device, in addition to the optical correcting system.
The beam spot detecting device detects the condensed state of a scanning beam, or the beam spot diameter or the amount of light corresponding to the beam spot diameter in a position substantially equivalent to the surface to be scanned, and determines the degree of a focal position deviation.
The displacing mechanism displaces one or more optical elements constituting the optical correcting system in an optical axis direction. When there are two or more optical elements to be independently displaced in the optical correcting system, each optical element is provided with the displacing mechanism. As the displacing mechanism, a heretofore known appropriate linear displacing mechanism using for example, a rack and pinion, a screw rod, or other such moving mechanism, can be preferably used.
The control device controls the displacing mechanism in accordance with a detection result of the beam spot detecting device, and corrects/adjusts the focal position deviation of a scanning beam on the surface to be scanned.
According to still another preferred embodiment of the present invention, in the immediately above-described optical scanning apparatuses, the correcting/adjusting device can be constituted to adjust the focal position in at least one of the main scanning direction and the sub scanning direction, to independently adjust the focal positions with respect to the main scanning direction and the sub scanning direction, or to simultaneously adjust the focal positions of the main scanning direction and the sub scanning direction.
When the correcting/adjusting device is arranged to adjust the focal position in at least one of the main scanning direction and the sub scanning direction, the optical correcting system may include a cylindrical lens which has a power in the main scanning direction or in the sub scanning direction, and the cylindrical lens can be displaced in the optical axis direction by the displacing mechanism.
When the correcting/adjusting device is arranged to independently adjust the focal positions with respect to the main scanning direction and the sub scanning direction, a lens for converting a light flux from the light source to a condensed light flux may be used as the coupling lens. Also, the optical correcting system may include a concave cylindrical lens having a negative power in the main scanning direction and a convex cylindrical lens having a positive power in the sub scanning direction. The displacing mechanism may be arranged to displace the concave cylindrical lens and the convex cylindrical lens independent of each other in the optical axis direction. Moreover, the optical scanning apparatus can be constructed so that a lens for converting a light flux from the light source to a divergent light flux is used as the coupling lens, the optical correcting system includes a convex cylindrical lens having a positive power in the main scanning direction and a convex cylindrical lens having a positive power in the sub scanning direction, and the displacing mechanism displaces the convex cylindrical lenses independent of each other in the optical axis direction.
When the correcting/adjusting device is arranged to simultaneously adjust the focal positions of the main scanning direction and the sub scanning direction, the optical correcting system may include a toroidal lens having a toroidal surface which is concave in the main scanning direction and convex in the sub scanning direction, and the displacing mechanism may be configured to displace the toroidal lens in the optical axis direction.
According to still another preferred embodiment of the present invention, in the above-described optical scanning apparatuses, the optical correcting system may have at least one special toroidal surface in order to effectively correct a wave-front aberration.
In recent years, the density of writing by optical scanning has been advanced, and a high-density writing exceeding 1200 dpi is being used. In order to accomplish the high-density writing, the beam spot diameter needs to be reduced, and a high NA is necessary for the optical system. However, when the high NA is realized, the beam diameter of a light flux flowing through the optical system is enlarged, and the wave-front aberration generated during the passage through the optical system largely influences the beam spot diameter. If such wave-front aberration is too large, the beam spot cannot be converted to a required small diameter.
In the optical scanning apparatus of preferred embodiments of the present invention, when the optical correcting system includes at least one special toroidal surface having a non-arc shape in the main scanning direction and/or the sub scanning direction, the wave-front aberration is effectively corrected, and a small-diameter beam spot is achieved reliably and continuously.